A sulphated galactan (SG) with low molecular weight (app. 2800) was isolated from extracts of Cryptopleura ramosa, a red seaweed from the South American coasts. The compound was a selective inhibitor of HSV-1 and HSV-2 replication in Vero cells with 50% inhibitory concentrations (IC50) in the range 1.6-4.2 micrograms/ml and a 50% cytotoxic concentration (CC50) of 476 micrograms/ml. SG was also effective against HSV-1 in cells of neural origin such as murine astrocytes. The mode of action of SG could be ascribed to an inhibitory action on virus adsorption. Furthermore, SG did not inhibit the blood coagulation process at concentrations highly exceeding the IC50.